The present lecture is the third in a series of eight lectures on neutrino physics that Rudolf Mößbauer held at the Lindau Meetings between 1979 and 2001. Since the audiences of students and young reserachers at the meetings change from year to to year, all his lectures start with a pedagogic historical introduction. This time he also had a lot of results to describe and thus the lecture is both unusually long, longer than one hour, and delivered in a tempo which I am sure that many in the audience had difficulties following. Not to mention the poor man who provided a simultaneous translation into English for the non-German speaking part of the audience! Mößbauer’s historic introductions change from lecture to lecture, much depending on the progress that was made between lectures. In particular the topic of the rest mass of the neutrinos was a hot topic in those years and a large number of experiments were ongoing. I remember this well from my time at the Research Institute for Physics in Stockholm, where K.E. Bergkvist made very precise measurements of the energy of the electrons emitted from the beta decay of tritium. We used to discuss his work almost every lunch. At the time of the present lecture, the observation of a third family of leptons had been accepted, but for simplicity Mößbauer still largely restricts himself to two families, the electron and the muon neutrinos. Since about 5 years he had been trying to detect oscillations between these types of neutrinos, an effect which theory predicted if the rest mass of the neutrinos was non-zero. For the first couple of years, the experiment was done at the ILL research reactor in Grenoble, but was then moved to a commercial reactor at Gösgen in Switzerland. The detector volume was only about 1 kubic meter, but the shielding was very heavy, and since the detection distance varied from 38 to 65 meters, there was an engineering problem to be solved. The results were published in 1986 and were consistent with the absence of neutrino oscillations. We now know that neutrino oscillations occur and that the rest mass of the neutrinos is non-zero. But this result has only been reached by using much larger detection volumes. Mößbauer ends his lecture by discussing the relevance of the neutrino experiments for elementary particle physics, astrophysics and cosmology. It is quite clear that in his opinion, the experiments are highly relevant, which also may explain his continuing interest and involvement with neutrino experiments. His final remarks concern the plans for an experiment named Gallex in the Gran Sasso tunnel north of Rome, which he describes as a beautiful place! Anders Bárány